The present invention relates generally to vehicles and more particularly to heavy duty industrial vehicles utilized as earthmoving equipment.
One type of vehicle that has been used extensively in large earthmoving equipment is known as an articulated vehicle. This articulated vehicle consists of two vehicle sections that are pivotally interconnected through a vertical pivot and the sections are pivoted relative to each other through hydraulic fluid motor means to control the turning movements for the vehicle. One type of steering system for producing the turning movements includes hydraulic rams which are pivotally connected between the respective frame sections and actuation of the rams will produce relative angular movement of one frame section with respect to the other frame thereby steering the vehicle. The most common steering system that has been utilized in articulated vehicles includes a conventional steering wheel attached to a steering column with a steering valve operated by the steering column. The steering valve in turn controls the flow of hydraulic fluid to and from a pair of hydraulic fluid rams that define the hydraulic motor means with the rams pivotally connected to the respective sections.
One type of commercial vehicle that is presently available is a Model W24B articulated loader that is being marketed by the assignee of the present invention. Because of the size of such a vehicle, which is used as heavy construction and earthmoving equipment, the fluid rams must be provided with sufficiently high input flow rates and pressure to provide the required steering system response for the articulated vehicle. However, this has presented some problems in the operation of the unit. One particular problem that has been pinpointed is the fact that with the large required flow rate of the hydraulic fluid in a system of this type, undesirable vibrations can be unexpectedly developed when steering is initiated or terminated.
In commercial heavy duty earthmoving equipment of the type mentioned above, the operator is seated on either the forward or rear section of the vehicle and is spaced from the pivotal point of that section (the axle) by a substantial distance. Thus, during turning movements of the vehicle, the operator's seat is actually laterally shifted when the vehicle changes direction.
It has been found that hydrostatic steering systems of the type incorporated into existing vehicles are capable of being actuated and neutralized very rapidly which results in what is referred to as "jerk" experienced by the operator.
This problem of "jerkiness" experienced by the operator becomes even more acute when the articulated vehicle is being transported at high rates of speed. It has been found that when a vehicle is travelling at a high rate of speed and encounters extremely rough terrain, such as railroad tracks on highways, it is extremely difficult for the operator to prevent inadvertent actuation of the steering system in opposite directions almost instantaneously which results in a swaying motion of the operator's seat in opposite directions.
Various attempts have been made to reduce the "jerk" experienced by the operator when a steering system is activated or neutralized, and examples of the prior attempts were primarily involved in the hydraulic system for the steering mechanism. Examples of such devices are time delayed devices, cross-over relief valves, pressure compensating valves and dumping valves incorporated into the hydraulic circuit for the steering system. One example of a prior attempt to minimize the "jerk" problem is disclosed in U.S. Pat. No. 3,959,968 which discusses the particular problem in terms of vibratory instability in the motion of a vehicle during turning movement. This patent proposes the solution in the form of a dampener associated with the steering valve that forms part of the hydrostatic circuit.